1. Field of the Invention
This invention pertains to compression wave generation. Specifically, the present invention relates to a device and method for directly generating sonic and ultrasonic compression waves, and indirectly generating a new sonic or subsonic compression wave by interaction of two ultrasonic compression waves having frequencies whose difference in value corresponds to the desired new sonic or subsonic compression wave frequencies.
2. State of the Art
Many attempts have been made to reproduce sound in its pure form. In a related patent application under Ser. No. 08/684,311, a detailed background of prior art in speaker technology using conventional speakers having radiating elements was reviewed and is hereby incorporated by reference. The primary disadvantage with use of such conventional speakers is distortion arising from the mass of the moving diaphragm or other radiating component. Related problems arise from distortion developed by mismatch of the radiator element across the spectrum of low, medium and high range frequenciesxe2x80x94a problem partially solved by the use of combinations of woofers, midrange and tweeter speakers.
Attempts to reproduce sound without use of a moving diaphragm include technologies embodied in parametric speakers, acoustic heterodyning, beat frequency interference and other forms of modulation of multiple frequencies to generate a new frequency. In theory, sound is developed by the interaction in air (as a nonlinear medium) of two ultrasonic frequencies whose difference in value falls within the audio range. Ideally, resulting compression waves would be projected within the air as a nonlinear medium, and would be heard as pure sound. Despite the ideal theory, general production of sound for practical applications has alluded the industry for over 100 years. Specifically, a basic parametric or heterodyne speaker has not been developed which can be applied in general applications in a manner such as conventional speaker systems.
A brief history of development of the theoretical parametric speaker array is provided in xe2x80x9cParametric Loudspeakerxe2x80x94Characteristics of Acoustic Field and Suitable Modulation of Carrier Ultrasoundxe2x80x9d, Aoki, Kamadura and Kumamoto, Electronics and Communications in Japan. Part 3, Vol. 74, No. 9 (March 1991). Although technical components and the theory of sound generation from a difference signal between two interfering ultrasonic frequencies is described, the practical realization of a commercial sound system was apparently unsuccessful. Note that this weakness in the prior art remains despite the assembly of a parametric speaker array consisting of as many as 1410 piezoelectric transducers yielding a speaker diameter of 42 cm. Virtually all prior research in the field of parametric sound has been based on the use of conventional ultrasonic transducers, typically of bimorf character.
U.S. Pat. No. 5,357,578 issued to Taniishi in October of 1994 introduced alternative solutions to the dilemma of developing a workable parametric speaker system. Hereagain, the proposed device comprises a transducer witch radiates the dual ultrasonic frequencies to generate the desired audio difference signal. However, this time the dual-frequency, ultrasonic signal is propagated from a gel medium on the face of the transducer. This medium 20 xe2x80x9cserves as a virtual acoustic source that produces the difference tone 23 whose frequency corresponds to the difference between frequencies f1 and f2.xe2x80x9d Col 4, lines 54-60. In other words, this 1994 reference abandons direct generation of the difference audio signal in air from the face of the transducer, and depends upon the nonlinearity of a gel medium to produce sound. This abrupt shift from transducer/air interface to proposed use of a gel medium reinforces the perception of apparent inoperativeness of prior art disclosures, at least for practical speaker applications.
It is an object of the present invention to provide a method and apparatus for indirectly emitting new sonic and subsonic waves at acceptable volume levels from a region of air without use of conventional piezoelectric bimorph transducers as the ultrasonic frequency source.
It is another object to indirectly generate at least one new sonic or subsonic wave having commercially acceptable volume levels by using a thin film emitter which provides interference between at least two ultrasonic signals having different frequencies equal to the at least one new sonic or subsonic wave.
It is still another object to provide a thin film speaker diaphragm capable of developing a uniform wave front across a broad ultrasonic emitter surface.
A still further object of this invention is to provide an improved speaker diaphragm capable of generating compression waves in response to electrical stimulation, yet which does not require a rigid diaphragm structure.
These objects are realized in a speaker which includes a thin, piezoelectric membrane disposed over a common emitter face having a plurality of apertures. The apertures are aligned so as to emit compression waves from the membrane along parallel axes, thereby developing a uniform wave front. The membrane is drawn into an arcuate configuration and maintained in tension across the apertures by a near vacuum which is created within a drum cavity behind the emitter membrane. The piezoelectric membrane responds to applied voltages to linearly distend or constrict, thereby modifying the curvature of the membrane over the aperture to yield a compression wave much like a conventional speaker diaphragm. This configuration not only enables compression wave generation, but also eliminates formation of adverse back-waves because of the applied vacuum.
In another aspect of the invention, the emitter includes a drum comprised of a single emitter membrane disposed over a support plate having a plurality of apertures at a common emitter face. The membrane is arcuately distended within the apertures for manipulation of the membrane in response to applied voltage.
In still another aspect of the invention a microphone device is developed by disposing a piezoelectric film as a detector membrane across apertures within a sensor face. This membrane, when in tension based on pressure applied from the drum cavity, is able to sense sound as compression waves. This is accomplished by the reverse process of the speaker embodiment referenced above, as electrical signals are generated within the piezoelectric material in response to impact of compression waves on the piezoelectric film.
Other objects, features, advantages and alternative aspects of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description, taken in combination with the accompanying drawings.